1. Field of the Invention
The present invention relates to an improvement in the aseptic packaging of fluid dairy products, and to the packaged products themselves. Fluid dairy products packaged in accordance with the invention are characterized by improved stability and improved flavor, generally.
2. Background
Fluid dairy products such as milk, cream, chocolate milk, and other such dairy products, can be preserved for short periods of time by refrigeration. At room termperature and even under refrigeration, these products deteriorate due to microbiological activity. This activity causes physical changes and leads to unacceptable flavor characteristics. Pasteurzation slows down and retards this activity but does not prevent it, it just postpones it.
Within the past 30 years, aseptic packaging systems have been developed to provide commercially feasible packaging of sterile dairy products intended for long term storage without refrigeration. These systems make use of ultra-high temperature (UHT) processing. UHT processing produces a product that is free of spoilage organisms by heating the fluid dairy product to a temperature that is high enough to kill spore-forming organisms, for a sufficiently short period of time so as to minimize the physical and chemical changes in the product itself. Common process parameters for UHT processing are a temperature in the range of 140.degree. C. to 150.degree. C. for a time 2 to 7 seconds. By utilizing the UHT processing in conjunction with an aseptic filling system, packaged fluid dairy products can be produced that remain fresh at ambient temperature for extended periods. The techniques and equipment that are required for UHT processing and for aseptic filling are well known in the art.
Two different types of UHT processing are in common usage. These are commonly referred to in the industry as the direct process and the indirect process.
In the direct sterilization process, steam is injected directly into the milk or other fluid dairy product to heat it rapidly with maximum efficieny of heat exchange. For a description of one direct sterilization process and the equipment used, see U.S. Pat. No. 3,230,095, the disclosure of which is incorporated herein by reference. Since the injected steam condenses directly in the milk, the condensation of the steam dilutes the milk with from 10% to 12% by weight of added water. This water must be removed prior to sale. Removal of the is generally accomplished by vacuum evaporation.
In the indirect sterilizaiton process, the milk or other liquid dairy product is passed through a heat exchanger, generally a heat exchanger of the tubular or plate types. During this heat exchange process, milk is generally passed through a tubular coil that is maintained in a very hot environment, often through the use of superheated steam. Because the heat exchange takes place through the wall of the tube, the heat exchange is less efficient than in the direct process, and the high temperature enviroment about the tube generally is at a very high temperature. This tends to produce a "cooked" flavor in the milk, as is well known among those in the industry.
To a lesser degree, the direct process also causes an undesirable flavor change. It is theorized that the undesirable flavor change is caused by the formation of free sulfhydryl materials in the milk during the UHT treatment. In addition, UHT-processed milk tends to exhibit some instability on standing. It is theorized that this instability, which manifests itself as fat separation among other things, is most likely caused by a change in the physical relationship between the fat, casein, and denatured serum proteins in the milk because of the UHT processing.